This application claims priority of European Patent Application EP01811216.9, the content of which is hereby incorporated.
1. Field of the Invention
The present invention concerns a measuring machine, notably a column for measuring longitudinal dimensions, for example a height-measuring column. The present invention also concerns a motorized driving device that can be used in such a measuring column.
2. Related Art
Height-measuring columns are described for example in document U.S. Pat. No. 4,924,598. They are used for example for measuring or comparing dimensions, for example in mechanical workshops. A measuring column generally comprises a fixed supporting frame with a base, a carriage that can be displaced vertically along the supporting frame, a device for driving the carriage and a system for measuring the carriage""s vertical position. A probe tip is connected to the carriage and is designed for being brought into contact with the piece to be measured. Some measuring columns comprise a base provided with means for creating an air-cushion in order to easily displace the height-measuring column on the work surface.
The measuring column described in U.S. Pat. No. 4,924,598 comprises an electric motor lodged in the base and driving the lower pulley through an axle or transmission belt. The lower pulley drives a driving belt coupled to the carriage, as well as a counterweight moving in opposite direction from the carriage. The driving belt is tensed between the lower and the upper pulleys. The carriage comprises wheels for pressing on the guiding rails attached to the supporting frame.
The electronic measuring system allows the position of the carriage, and thus of the probe tip, to be determined and displayed on an electronic display. The resolution and precision that is expected of this type of measuring columns is on the order of the micron.
This precision depends for an important part on the contact force between the probe tip and the piece to be measured. A substantial contact force causes a flexion of the probe tip and/or of the piece, or even an elastic deformation of the material, that can influence the measuring. The contact force between the probe tip and the piece to be measured must thus be minimal or, in any case, identical at each measuring.
It is thus essential to realize the driving system so that the traction force exerted on the carriage should be reproducible and independent from the carriage""s longitudinal position. For this purpose, it is important to make sure that the driving belt is sufficiently tensed so that it does not slide on the driving pulley. A significant tension will however cause forces and moments on the carriage that can vary according to the carriage""s position and thus influence the measuring. In order to absorb these constraints and reduce the play, it is necessary to reduce as much as possible the play between the carriage""s wheels and the guiding rails on the supporting frame. A strong pressure between the carriage""s wheels and the rails increases however the wheel""s resistance, which makes displacing the carriage difficult and even noisy. A significant wheel resistance furthermore causes an even greater traction force of the driving belt, which further increases the constraints exerted by the driving belt on the carriage and on the probe tip.
Furthermore, the space requirement and the weight of the measuring column should be reduced and the number of distinct parts should be limited. A heavy and cumbersome column, constituted of many distinct elements, is difficult to operate, expensive to make and transport and is more likely to break down and to malfunction.
Furthermore, the use of a belt between the motor and the driving pulley is an additional source of friction and play, which is detrimental to the accuracy of the driving and thus to the measuring accuracy. It is not possible for the pulley to be directly driven by the motor when the carriage can be moved by hand by the user; during sudden displacements, the motor could be damaged by the generated electric tension.
It is an aim of the present invention to propose a column for measuring longitudinal dimensions that avoids the disadvantages of the prior art columns. In particular, it is an aim of the present invention to make a column for measuring longitudinal dimensions in which the carriage""s driving belt or cable is less tensed than in the prior art devices and whose total space requirement is reduced.
According to the invention, these aims are achieved by means of a measuring column comprising the characteristics of claim 1, preferred embodiments being furthermore indicated in the dependent claims.
In particular, these aims are achieved in that the motor driving the driving belt is placed in the upper part of the supporting frame.
This has the advantage that the belt""s tension at the upper driving pulley is produced only by the carriage""s mass and the counterweight, without additional tensors being necessary. The tension at the lower pulley being not critical, it is possible to use driving belts less tensed than in the prior art.
These aims are further achieved by means of a motorized driving device comprising a pulley, a driving motor and a friction element, in which the driving motor and the friction element are integrated inside the pulley.
This characteristic has the advantage of allowing a construction that is extremely compact and light and also economical, as will be seen.